The present invention is related to engine control systems which implement engine idle speed control. More specifically, the present invention is related to engine control systems which develop electrical control signals that are utilized to control engine idle speed.
Engine control systems are known in which an air bypass valve (dashpot) is provided such that under certain conditions, such as deceleration, additional air is provided to the engine fuel mixture. Such systems recognize that the idle speed of an engine bears a direct such an air bypass valve. One such system is disclosed relationship to the amount of additional air provided by in U.S. Pat. No. 4,453,514, entitled, "Engine Speed Adaptive Air Bypass Valve (Dashpot) Control", which is assigned to the same assignee as the present invention.
Some engine control systems have utilized the known relationship between the degree of air bypass valve actuation and idle speed to implement an idle speed control system in which a desired idle speed is calculated as a function of engine coolant temperature and the degree of bypass actuation is a function of the difference between actual engine speed and this desired engine idle speed. In these known systems, typically the control of idle speed is implemented in direct proportion to the difference between actual engine speed and desired idle speed. In some systems, the idle speed control is implemented by the enabling of a closed loop servo-control system when engine speed approaches the desired idle speed.
Typically, such engine idle speed control systems do not provide servo-control in accordance with the integral of the difference between actual engine speed and desired idle speed, as well as in proportion to the difference between actual and desired idle speed. By controlling idle speed in accordance with the integral of the difference between actual speed and desired idle speed during closed loop operation, an engine idle speed control system will reduce idle speed overshoot error by preventing the idle speed control from too rapidly responding to transient conditions. This will prevent engine stalls from occuring during transient conditions. However, providing an integral of the difference between actual speed and desired idle speed would typically slow the response of the idle speed control system thus lessening the desirability of the use of such a system. In addition, typically the prior idle speed control circuits are separate from other engine control circuits, thus requiring extra cost for the total engine control system.